This invention relates to operational amplifiers, and in particular to fast operating devices of that kind suitable for use in high speed, data-handling systems.
Operational amplifiers are often used as buffer amplifiers or voltage followers in high speed data handling systems and are a major factor in limiting system speed. Most commercially available operational amplifiers require several gain stages in order to achieve useable open loop voltage gain.
The design of any integrated circuit operational amplifier involves a compromise between voltage gain and slew rate (or bandwidth). Some high DC gain-high slew rate operational amplifiers are available in hybrid form which use large value feed-forward capacitors to change the effective circuit configuration at high frequencies. A typical state-of-the-art device is the Harris Semiconductor, Model HA 2520/2522/2525 operational amplifier which uses two gain stages with a Darlington input. The circuit achieves a slew rate of 120V/.mu. sec. by operating at a large amount of power (300 mW) and is unity gain stable. Optical Electronics, Inc. produces a fast hybrid operational amplifier designated as their model 9808. This amplifier has a slew rate of 2000V/.mu. sec. but dissipation up to 900 mW of quiescent power. It uses a standard integrated circuit to obtain high-low frequency gain and feed-forward to bypass the integrated circuit to obtain high speed operation. National Semiconductor uses a simple one gain stage circuit with FET inputs to achieve 500V/.mu. sec. in their LH0032/LH0032C "ultra fast operational amplifier" . These amplifiers, however, are not unity gain stable and require large amounts of power (1.5 watts at 25.degree. ambient). Hybrid amplifiers of the type exemplified by these and other state-of-the-art devices are bulky, expensive, and consume high power. Accordingly, there currently exists the need for an inexpensive low power operational amplifier having slew rates comparable to hybrid devices. The present invention is directed toward satisfying that need.